Compounds having isocyanate group(s) exhibit excellent properties as a crosslinking agent. However, these compounds have some disadvantages such that they are liable to react with water, bases, acids, ammonia, amines, alcohols, phenols, carboxylic acids and acid anhydrides at room temperature and that it is difficult to interrupt the above reactons once they start. In order to overcome these disadvantages, there have been proposed processes wherein an isocyanate compound is tentatively reacted with an appropriate compound such as an amine, acidic sulfite, tertiary alcohol, oxime, prussic acid, boric acid, phenol, mercaptan or enol lactam to thereby give a product in the form of an isocyanate regenerating material or an ioscyanate adduct, which is heated to regenerate or dissociate the original isocyanate compound when used as a crosslinking agent (cf. Japanese Patent Publication No. 30846/1980 and No. 41841/1983). However, these processes have some disadvantages in that the regeneration step as described above requires a high temperature of 160.degree. to 180.degree. C. and that the regeneration ratio of the isocyanate regenerating material falls upon storage.